This invention relates generally to a process for the production of glycols and, more specifically, to a catalyzed continuous process to convert carbohydrates to mainly either ethylene glycol or propylene glycol.
Ethylene glycol is a valuable commodity chemical that has a broad range of uses as both a building block for other materials such as polyethylene terephthalate (PET) and for its intrinsic properties such as for antifreeze. It is currently made by a multistep process that starts with ethylene derived from hydrocarbon feedstocks.
A cost-effective way to produce ethylene glycol from renewable resources would reduce dependence on non-renewable hydrocarbon feedstocks and create substantial new uses for agricultural based products. Several patents have demonstrated that carbohydrates, one of the most abundant renewable resources, can be converted to ethylene glycol.
Early approaches have been based on the use of somewhat non-selective hydrogenolysis for the conversion of carbohydrates to ethylene glycol. As an example U.S. Pat. No. 5,210,335 describes a reaction system with a high catalyst loading that produces 20 wt. % ethylene glycol and 60 wt. % propylene glycol plus a range of other components. EP2419393 “Method for the hydrogenolysis of sugar alcohols” has reduced the required catalyst concentration, but still produces a range of products with ethylene glycol being only about 8-12 mole % of the final product. These approaches will require extensive post reactor separations and markets for the various co-products to be cost effective.
Recent work described in U.S. Pat. Appl. 2012/0172633 (Zhang, et al.) using a low catalyst loading has demonstrated a much higher selectivity to ethylene glycol by achieving a yield of greater than 50% to as high as 68% depending on the choice of feedstocks. However, the reactor concentrations demonstrated were not commercially viable at only 1% solution of the feedstock. A very recent paper has demonstrated that the reason for the higher selectivity is due to a different mechanism for tungsten based catalysts (Ooms, R., et al. Conversion of sugars to ethylene glycol with nickel tungsten carbide in a fed-batch reactor: high productivity and reaction network elucidation. Green Chem., 2014, 16, 695-707). Initially the tungsten very selectively converts aldohexoses such as glucose to glycolaldehyde and erythrose by a retro-aldol mechanism and then further converts the erythrose into two more glycolaldehydes. The glycolaldehydes are hydrogenated by another catalyst to the ethylene glycol. The problem in the Zhang application and the Ooms paper is that the writers try to do the whole reaction sequence in one process step which introduces unnecessary complexity to the catalyst composition and manufacturing process and as importantly generates more impurities.
In addition, all of the methods described above are batch or semi-batch which are not as cost-effective for commodity production processes. In addition the described methods operate in stirred reactors with very high agitation rates, which rapidly powders the solid catalyst particles. Powdered reduction catalysts can be a very major operating hazard in commercial scale production processes. This invention improves the selectivity to the desired glycol product, increases the processing concentrations to more commercially viable levels and demonstrates a safer, continuous process for the cost-effective production of glycols from carbohydrates.